Our studies on the mitogenicity and polyclonal activating properties of endotoxin protein have shown that this material can act as a potent stimulator of proliferation and nonspecific antibody production by the B lymphocytes of animals and humans. Endotoxin protein is found in the outer membranes of Gram-negative bacteria closely associated with lipopolysaccharide endotoxin. Analysis by SDS-polyacrylamide gel electrophoresis indicates it is composed of 4 to 5 major polypeptides depending on the organism from which it is isolated. Our objectives include the separation of these polypeptides to assess which ones are active on mouse and human cells, and to determine whether these polypeptides are found throughout the spectrum of the Gram-negative genera. Furthermore, we will determine whether endotoxin protein can act as an immunoadjuvant, an activator of human monocytes as well as lymphocytes, and be useful for monitoring B-cell function in immunodeficiency patients.